Method for growing single crystals of semiconductors



N. R. KYLE July 7, 1970 METHOD FOR GROWING SINGLE CRYSTALS OF SEMICONDULIOHS Original Filed Oct. 1, 1964 Nonse R. Kyie,

INVENTOR.

United States Patent 3,519,399 METHOD FOR GROWING SINGLE CRYSTALS 0F SEMICONDUCTORS Nansc R. Kyle, Long Beach, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Continuation of application Ser. No. 400,723, Oct. 1, 1964. This application May 22, 1967, Ser. No. 640,303 Int. Cl. B01d 9/00 US. Cl. 23-294 2 Claims ABSTRACT OF THE DISCLOSURE A method of simultaneous purification and growth of semiconductor material into a single stoichiometrically pure crystal by sublimation and condensation of the material by means of either a modified Bridgman process or by means of a vapor growth process. The semiconductor mateiral comprises the compounds selected from the Group II and VI elements of the periodic table, such as cadmium telluride. The charge material is vaporized in the upper part of a growth tube and is condensed in the lower part of the tube into first a liquid and then the crystal (Bridgman process) or directly into the crystal (vapor growth). A small container is secured in the upper part to contain the charge.

The present invention is a continuation of copending application, Ser. No. 400,723, now abandoned, filed Oct. 1, 1964, for Method for Growing Single Crystals of Semiconductors.

The present invention relates to a method of simultaneous purification and growth of semiconductor material into a single crystal by sublimation and condensation of the material by means of either a modified Bridgman process or by means of a vapor growth process. In particular, the semiconductor material comprises the compounds selected from the Group II and VI elements of the periodic table, such as cadmium telluride, for use in electronic semiconductor devices.

The well-known Bridgman method for growing a crystal from a charge material comprises the steps of placing the charge in a suitable crucible and clowly lowering the crucible and charge through a furnace having a temperature curve or profile, including a decreasing temperature gradient. The curve is provided with an initial portion which is above the melting point of the charge and a decreasing temperature or freezing gradient which passes through the melting point of the charge. Thus, the charge is first melted in the initial portion of the temperature profile and is subsequently solidified as the crucible and charge pass through the freezing gradient. The solidification progresses at a rate which corresponds to the speed of movement of the charge as it passes through the freezing gradient. stoichiometrically pure cadmium telluride, for example, which is cadmium telluride free from excess cadmium or excess tellurium, can be obtained by the use of the Bridgman method if the charge material in the tube is stoichiometrically pure. However, if the cad-mium telluride or other semiconductor charge material initially placed in the tube is not stoichiometrically pure, the crystals obtained by the use of the Bridgman method also generally are not stoichiometrically pure,

The present inventive method overcomes this and other problems by producing stoichiometrically pure crystals from stoichiometrically impure starting materials by enther a modified Bridgman method or a vapor growth method. Essentially, the charge material is physically segregated or separated from the growing crystal material. To achieve this result, a novel crystal growing tube is 3,519,399 Patented July 7, 1970 utilized. The tube comprises an upper portion having a cup and a lower portion terminating in a nucleation point. The starting or charge material is placed in the cup and the tube is evacuated and sealed. The sealed tube is then placed within a furnace provided with a specified temperature curve or profile. Depending upon whether a modified Bridgman method or a vapor growth process is used, the initial portion of the temperature curve is above the melting point of the charge material or is immediately below the materials melting point. The initial portion is above the melting point when the modified Bridgman method is used and below the melting point when the vapor growth method is employed.

With either method, the charge material vaporizes in the initial portion of the temperature profile portion of the furnace. As the tube is lowered through the furnace and the temperature profile, the nucleation point passes through further portions of the curve. These further portions comprise a decreasing temperature gradient, are at temperatures which are lower than that of the initial portion, and contain a freezing gradient at which point the vaporized charge stoichiometrically combines and ultimately forms into a stoichiometrically pure crystal.

In the modified Bridgmen method, the vaporized charge first condenses into a liquid which then solidifies as the tube passes through the freezing gradient. In the vapor growth method, the vaporized charge material directly condenses into a solid. The vapor growth method is advantageous when a low temperature growth of the crystal is desired.

The specific temperatures within the furnace are dependent upon the physical characteristics of the compound crystal and its ingredients. However, the present invention has been adaptable to the formation of crystals selected from the Group II and Group VI elements of the periodic table, for example, combinations of zinc, cadmium and mercury with sulpher, selenium and tellurium. Furthermore, the travel of the tube and the gradient of the temperature profile are controlled to permit growth of single crystals which are free from the presence of inversion twins and separation planes.

The method and tube are further arranged so that the influence of gravity may be utilized. Many crystal materials do not adhere to the tube and, therefore, it is not possible to place the nucleation point at the upper portion of the tube. In part because of this problem, many crystal growing and purification methods draw the tube horizontally through a furnace. However, it is not possible at all times to obtain pure and stoichiometrically pure crystals by means of the horizontal growing method. It is, in part, for this reason that the zone melting process (see Phann Pat. 2,739,088, for example) had been developed.

By means of the present invention, segregation of the starting charge from the crystal can be simply obtained by placing the charge material in the upper portion of the tube and the crystal growth portion in the lower portion of the tube to take advantage of the influence of gravity and to overcome any problems which may arise when the crystal does not adhere to the tube. In addition, the present invention also permits easy removal of the formed crystal from the tube because special procedures are not required to enhance adhesion of the crystal to the tube.

Therefore, the tube comprises an elongated cylinder having a nucleation point at one end and a cup or receptacle secured to the interior of the cylinder at the other end. The tube may be formed from any material which is nonreactive with the crystal materials and which can withstand the temperatures and pressures employed in the process. Quartz is preferred as the tube material;

3 however, any material which conforms to the above conditions is suitable.

Accordingly, it is a primary object of this invention to provide a method for producing stoichiometrically pure single crystals of compound semiconductor materials from starting materials that are not stoichiometrically pure.

Another object of this invention is to provide a method for producing pure crystals of semiconductor materials for use in electronic semiconductor devices.

Another object is the provision of a tube suitable for carrying out the method.

Other aims and objects, as well as a more complete understanding of the present invention will appear from the following explanation of an exemplary embodiment and the accompanying drawings thereof, in which:

The figure is an elevational view, in longitudinal section, showing a crystallization tube containing a cup of charge material mounted in the upper part thereof and the furnace utilized for growing a stoichiometrically pure crystal. In dashed lines, the nucleation tip of the tube is shown, after further downward movement of the tube through the furnace, with formed crystals therein after the tip has passed through the crystal freezing point of the furnace.

Stated in general terms, the objects of the invention are attained by placing the charge material, such as cadmium telluride, in the upper part of an elongate crystallization tube having a pointed lower end for crystal nucleation, sealing the tube, and slowly lowering the tube through a heated zone having a decreasing temperature gradient, such as is provided by a furnace, while maintaining the upper part of the tube hotter than the lower part. In the modified Bridgman embodiment, the upper part is maintained at a temperature above the melting point of the charge material so that the charge material vaporizes from the upper part of the tube and condenses in the lower part of the tube into a liquid or melt as the tube passes through the gradient. The condensed liquid in the lower part of the tube solidifies into a single crystal, as in the Bridgman method, as the tube further passes through the furnace. Alternatively, in the vapor growth embodiment, the charge material is heated to a temperature which is lower than its melting point so that a crystal may be formed by vapor growth. In either case, the charge material is placed in the upper part of the tube and the crystal is formed in the lower part thereof.

Accordingly, a tube formed from quartz, for example, comprises an elongated body having a nucleation tip 12 terminating in a point 14 at its lower end 16 and an upper portion 18. A quartz cup or receptacle 20 is positioned within the tube at its upper end and sealed to the interior surface by supports 22. A lowering rod 24 is secured to upper end 18.

Tube 10 is adapted to be placed within a furnace 26 which is closed by a lower cover 28 and an upper cover 30. An opening 32 is placed within cover 30 to receive rod 24. The furnace is provided with a temperature curve or profile 34 having an initial portion 36 and lower temperature portions 38 including a decreasing temperature gradient. The initial portion has a length which at least equals the length of tube 10 and, as shown, is an isothermal portion of profile 34.

In operation, a charge 40 of starting material, such as cadmium telluride, is placed within cup 20. The tube is then evacuated and sealed.

Charged, evacuated and sealed tube 10 is lowered into flat isothermal portion 36 of the temperature profile of the furnace. In the modified Bridgman embodiment, portion 36 is above the melting point of the charge and is retained there for about one to two hours to permit the temperature to stabilize. The tube then is lowered through the furnace at a rate commensurate with the vaporization of the charge and formation of the crystal until the tube clears the furnace. During the movement through the furnace, charge 40 vaporizes from cup 20 and collects at the tip or point, as indicated by indicia 12', of tube 10 in a stoichiometrically pure form of a liquid or melt as tip 20 passes through gradient 38. As the collected liquid in the tip passes through lower portion 38 which contains the freezing point of the temperature curve, a crystallite 42 nucleates at tip 12' and subsequently the crystallite grows as tube 10 is slowly moved further through the furnace and as additional vapors condense and crystallize.

In the vapor growth embodiment of the present invention, the maximum portion of the temperature profile of the furnace is maintained at a temperature which is below the melting point of the starting material so that, as tube 10 and the charge is lowered through the furnace, the charge vaporizes and then solidifies into crystal 42 at tip 12' without passing through a liquid phase.

The embodiments of the inventive method yield a stoichiometric composition since a crystal material, such as cadmium telluride, vaporizes according to the equation:

The decomposition vapor formed in the hotter portion of tube 10 consists of equal atomic amounts of cadmium and tellurium, which recombine in the cooler growth portion 14 in stoichiometric proportions. Selfnucleated crystal growth commences in tip 12 of tube 10 from the vapor phase, and slowly continues until a purified crystal of controlled stoichiometry of cadmium telluride results. This process can be repeated as often as desired.

"EXAMPLE 1 A stoichiometrically pure single crystal of cadmium telluride was prepared from a charge of cadmium telluride containing only approximate stoichiometric quantities of cadmium and tellurium.

The combination of cadmium and tellurium was placed in cup 20. The tube was evacuated to a pressure of 10- torr. and sealed. The tube was then placed within furnace 26 at the end of rod 24 and the temperature of portion 36 was raised to 1100 C., which is above the melting point 1093 C. of cadmium. telluride. The tube remained stationary within the furnace for approximately one hour to permit the temperature of the furnace, the tube and the starting material to achieve the equilibrium temperature of 1100 C. and to vaporize the charge. The tube was then lowered through the furnace and the ternperature curve at approximately 0.5 cm./hour to cause the vapors to condense first into a liquid and then into a solid.

EXAMPLE 2 The same conditions as described in Example 1 were employed for cadmium telluride with the exception that initial portion 36 of curve 34 was set at approximately 1050 C. The charge vaporized in portion 36 and then condensed directly into a solid crystal in portion 38.

In both examples, a stoichiometrically pure single crystal of cadmium telluride was obtained.

The use of the method of the invention applies, in general, to any compound or element that can be vaporized and condensed. Other semiconductor compounds of cadmium, such as cadmium selenide, etc., also can be grown to produce stoichiometrically pure single crystals by the use of the method of the invention. In general, compounds of the II-VI type of the periodic table, which can be vaporized and condensed, can be processed in accordance with the method of the invention to produce stoichiometrically pure single crystals.

Although the invention has been described with reference to a particular embodiment thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of growing stoichiometrically pure single crystals from a charge material in a vertical furnace by means of a modified Bridgman process, the furnace having means providing a temperature profile having a first temperature portion at the upper part of the furnace for vaporizing the charge material and a decreasing temperature gradient portion at the lower part of the furnace decreasing from a point adjacent the first portion, comprising the steps of:

placing the charge material in the upper part of a crystallization tube,

evacuating and sealing the tube and placing the tube in the upper furnace part,

maintaining the first temperature portion above the melting point of the charge material so that the material melts and vaporizes, and maintaining the gradient portion at the melting point and below the melting point of the charge material,

lowering the tube and the charge material through the furnace from the first temperature portion and through the temperature gradient portion at a rate commensurate with crystal growth so that the vaporized charge material in the lower tube part passes first into a liquid phase and then stoichiometrically crystallizes, and

recovering the single crystals from the tube.

References Cited UNITED STATES PATENTS 2,947,613 8/1960 Reynolds 23294 2,657,122 10/1953 Chaudoze 23301 3,210,149 10/1965 Eland 23-301 FOREIGN PATENTS 894,739 1/ 1961 Great Britain.

OTHER REFERENCES Lawson et al., Preparation of Single Crystals. (pp. 14-17) Butterworth Scientific Publications, 1958, London.

NORMAN YUDKOFF, Primary Examiner US. Cl. X.R. 23301 

